JPH11112070A - Fiber laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fiber laser wherein Δ of a core is kept low to prevent a rare earth dopant from re-crystallizing, for increased diameter of an effective core. SOLUTION: Relating to a certain length glass fiber comprising a core 21, a first clad layer 22, and a second clad layer 23, the composition of the core is: 0.1-2.0 mol.% of rare earth element; 0.2-3.0 mol.% of Ge; 0.5-8.0 mol.% of Al; 0.5-8.0 mol.% of P; and SiO2 of remaining portion, while the difference in index of refraction between the core 21 and the first clad layer 22 is 0.008 or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a fiber laser device for pumping a laser through a cladding layer.

[0002]

BACKGROUND OF THE INVENTION Rare earth doped fiber lasers are widely used in optical communication systems integrated with active fiber devices such as fiber links and erbium-doped fiber amplifiers. Fiber lasers are lasers that are pumped with GaAlAs, high power, inexpensive multimode lasers with single mode output. Fiber lasers have a large active area, so that there is almost no heating effect (which adversely affects the life of other semiconductor laser structures). In this regard, for example, L. “High-Po” by Zenteno
wer Double-CladFiber Lasers ”, Journal of Lightwav
e See Technolory, Vol. 11, No. 9, pp. 1435-1446, Sept. 1993.

[0003] The power of a fiber laser is proportional to the cavity length. However, the major loss in the fiber material is proportional to its length, and the loss increases over time, destabilizing system performance. Another preferred way to increase power is to increase the core diameter.
That is, to increase the active core area by increasing the pump absorption area over a fiber length.
However, in single mode output, it is necessary to reduce Δ of the core. Maintaining a threshold level of Ge dopant is necessary to write a Bragg grating in the fiber core, thereby forming a laser cavity. At the same time, it is also known that the need for dopants such as Al is needed to melt the active rare earth ions. For this reason, in the absence of a certain effective amount of Al, the rare earth dopant crystallizes and causes excessive scattering losses. However, both of these additives (Al, Ge) increase the Δ of the core. The concentrations of Ge and Al to keep the overall level of refractive index altering dopant low enough to satisfy the Δ of the core described above are too low to achieve the above objective (anti-recrystallization).

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to meet these contradictory requirements (keeping the core Δ low and preventing recrystallization of rare earth dopants) and to increase the effective core diameter. Seeking an approach.

[0005]

SUMMARY OF THE INVENTION According to the composition of the core of the optical fiber laser of the present invention, a large amount of a plurality of refractive index modifying dopants is contained in the core, so that their sum has an influence on the refractive index of the core. The effect is to be less than the effect of changing the refractive index of only the individual components. The diameter of the core of the fiber laser can be greatly increased by the composition of the core of the optical fiber of the present invention. Such a synthetic effect is obtained by using P as a counter-effect dopant, which counteracts the effect of at least one refractive index change of the essential constituent material (in this case Al).

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG.
And a part of the optical fiber coil 11 is cut to show a certain length. The length of the fiber in this structure is typically on the order of tens of meters, and the fiber is shown in the figure as being wound multiple times. The fiber is wound into mandrel structures of various shapes, such as circular or elliptical. Although in principle the fiber is shown as being pumped at the end, it may be pumped over the entire length of the fiber or a part thereof.

The laser cavity is formed by plug reflectors 12, 13 shown at both ends of the coiled fiber. These reflectors 12, 13 or gratings are created by forming a refractive index change in the core of the optical fiber with the illuminating light. Preferably, the core is exposed before writing the grating, for example by hydrogen sintering.
oak) to give appropriate photosensitivity. The desired grating pattern can be obtained by changing the intensity of light along the length of the optical fiber, that is, by changing the intensity periodically, by using actinic radiation (generally a frequency pumped near the wavelength of 240 nm of an excimer laser). (Dye laser). It is formed using a photomask or a buffer pattern using the generated patterned light beam. This refractive index variation is created by the refractive index variation introduced by UV at the defect sites of the absorbing ions such as Ge. The Ge-doped fiber becomes photosensitive by a known hydrogen or deuterium treatment. Very large (more than 0.01) refractive index variations can be obtained by such a process. These techniques for forming optical gratings are described, for example, in US Pat. No. 4,725,110.
It is disclosed and known in U.S. Pat. No. 5,327,515.

Referring again to FIG. 1, the fiber laser is pumped at the end by a laser diode 14. The output of the fiber laser is shown at 15. FIG. 1 is not shown to scale. For example, a laser diode
Although shown remote from the optical fiber, a laser may be connected to the optical fiber with a suitable coupler.

FIG. 2 is an end view of the optical fiber. The figure is a cross-sectional view at any position along the optical fiber. This fiber laser has a core 21, a first cladding layer 22, and a second cladding layer 23. The fiber laser is shown in circular cross section, but may be slightly elliptical to allow mode coupling. The composition of the core of the fiber according to the invention is as described below. The first cladding layer 22 is a high purity silica material, preferably pure SiO ₂ , or at least 85% SiO ₂ . Preferably, the first cladding layer 22 is, for example, Ge,
Includes dopants such as Al and P to increase the refractive index of the cladding layer and reduce Δ between the cladding layer and the core. In principle, this alleviates the aforementioned constraints. That is, the core can be heavily doped while maintaining the desired overall waveguide properties of the core / cladding layer. However, it is preferred to reduce Δ using the principles of the present invention. This is because suitably doped cladding materials of sufficient purity are not commercially available.

The second cladding layer 23 is formed of the first cladding layer 2
Any cladding material capable of confining the pump radiation within 2, ie having a sufficient Δ with respect to the first cladding layer 22. In the present specification, Δ is 0.
03 or more. The preferred second cladding layer 23 is one of the known polymer coating materials doped with fluorine to produce the required Δ. The advantage of choosing such a material is that the second cladding layer 23 can also function as the primary fiber coating. Examples of such suitable materials include UV-curable fluorinated acrylates.
crylates).

The dimensions of the structure shown in FIG. 2 can vary greatly. The first cladding layer 22 is usually in the range of 50 to 400 μm, preferably in the range of 100 to 300 μm. The thickness of the second cladding layer 23 is from 10 μm to several hundred μs.
m. This layer is relatively thin for light guiding purposes. If the second cladding layer 23 is the main or only coating layer, a greater thickness is desirable.

The size of the core is important for the present invention. The core size commonly used in the market is 6μ
m. It is important that the core diameters be closely matched to facilitate the connection of the fiber laser to standard input / output fiber links or other fiber devices and to reduce its splice loss. However, the diameter of the core is less than 6 μm in a general optical fiber having a core composition that enables UV writing of the grating. For example, the core of a fiber laser has the following composition (the composition is represented by a refractive index). Yb ^{+ 3} : Δn = 0.001 Al: Δn = 0.004 Ge: Δn = 0.004 Whole: Δn = 0.09

[0013] Yb is included at a concentration sufficient to absorb enough pump radiation to provide the desired output power level. This concentration exhibits an absorption of 150 dB / m at 915 nm. Al is contained to dissolve Yb. Ge is contained in an amount sufficient to write the Bragg grating.

For single mode operation at 1060 nm, the core size of this fiber laser is 4.7 μm. For a fiber with an overall diameter of 200 μm, the length of this fiber laser is 100 m or more.

According to the present invention, the core of the fiber laser is P
Is doped. P is contained in the core at a level sufficient to melt rare earth ions without increasing the Δ of the core, in opposition to the refractive index changing effect of Al. The general purpose is to set the Δ of the core to 0.007 or less using the above-described refractive index compensation technique (for the first cladding layer 22,
(Based on pure silica).
The following core compositions are given as examples of the present invention. In each case, the host material was silica, and the Δ of the core was calculated with silica as the first cladding layer 22.

Experimental Example 1 Composition mol% Δn Yb ⁺³ 0.5 0.002 Al 5.7 0.0054 P 1.4 0.0014 Ge 0.4 0.0006 Δn = 0.0066

The actual contribution to Δn from the combination of Al and P is 0.004. The reason is that these cancel each other out. The core diameter of this fiber is 5.0 μm and the cutoff wavelength is 950 nm or less. The diameter of the inner cladding layer is 200 μm and 915 n
m showed an absorption of 0.18 dB / m. The length of this device was about 70 m. These properties are
This example was preferred except that the Ge concentration of the device was too low to write a robust grating. If the concentration of Ge is increased to 1.5% for writing the grating and the concentration of the other composition is kept the same, the length of the device becomes 110 m.

In the following experimental example, the concentration of Ge is increased to facilitate writing of the grating into the device, and the concentrations of Al and P are balanced to offset the contribution to Δn. .

Experimental Example 2 Composition mol% Δn Yb ⁺³ 0.5 0.002 Al 4.3 0.004 P 4.3 0.004 Ge 1.5 0.0022 Δn = 0.0042

The diameter of the core of this fiber is 6.2 μm.
And the wavelength is 950 nm or less. The absorptivity of the 200 μm cladding layer is 0.28 dB / m, and the length of this device is 45 m, which is much shorter than the device of Experimental Example 1. Gratings can be easily written on this fiber. However, when writing a grating in a fiber of this composition, the writing conditions must be carefully controlled to avoid crystallization.

Experimental Example 3 Composition mol% Δn Yb ⁺³ 0.5 0.002 Al 5.7 0.0054 P 8.6 0.0081 Ge 1.5 0.0022 Δn = 0.0069

The diameter of the core of this fiber is 5.12 μm.
And the wavelength is 950 nm or less. The absorptance of the 200 μm cladding layer is 0.28 dB / m, and the length of this device is 66 m, which is significantly shorter than the device of Experimental Example 2. The structure of this fiber can be written with a strong grating and can be easily written without crystallization.

The rare earth used in most of these examples of the present invention is Yb and the output wavelength is 1060 nm. Other rare earths such as Ho, Nd, Er, Tm, Dy
Can also be replaced in whole or in part. The ionic concentration varies depending on the level of absorption of the pump radiation used, but is usually between 0.1 and 3.0 mol%, generally between 0.1 and 2.0 mol%. The amount of Al required to melt the rare earth elements in these compositions is 0.5 to
The content of P is within the range of 8.0 mol%, and the amount of P added to cancel the effect of the change in the refractive index of Al is 0.5 to 8.0.
Mol%. In order to write the Bragg grating efficiently, the amount of Ge required is 0.2 to 3.0.
Mol%. Fibers with these core dopants can be formed by mixing the constituent compositions in the form of their oxides during the manufacture of the fiber preform.

In addition to the above restrictions on the range of constituent compositions, the overall refractive index variation, or Δ of the core, must be minimal to achieve the objectives of the present invention. The combined mole% of the plurality of dopants should make the overall refractive index difference between the core 21 and the first cladding layer 22 less than 0.008, preferably less than 0.0072.

The pump diode used in these examples is a broadband GaAlAs device. However, other semiconductor laser pump sources, for example,
InGaAs or InGaAsP can also be used.
Semiconductor pump lasers are preferred, but other pump sources,
For example, Nd-glass and Ti-sapphire can be used.

The diameter of the core of the fiber of the fiber laser device used in these experimental examples is about 6 μm.
This is about 30% larger than the diameter of a conventional core, but this difference is more than 60% based on the effective area of the core exposed to pump radiation. Thus, for a given output power, the length of this fiber laser is less than half the length of the prior art fiber laser. Furthermore, the flags on the Bragg grating decrease at the same area ratio. Thus, during the life of the device, the possibility of reflector damage can be reduced. Using the teachings of the present invention, the diameter of the common fiber core (i.e., 5.5 μm to 7.5 μm)
A fiber laser having a core substantially conforming to m) can be easily manufactured.

In Experimental Example 2, the same composition of Al and P is used. That is, the amount is used to largely cancel the effect of changing the refractive index of each ion and minimize Δ of the core. In the present invention, it has been found that when equal amounts of Al and P are used, this is the most effective approach for minimizing the Δ of the core, but the composition of the rare earth element is easily crystallized. Investigations have shown that the degree of melting of the rare earth elements is more effective when either Al or P is used in excess. Furthermore, as shown in Experimental Examples 1 and 3, the molar amounts of Al and P are not equal. The excess of one (Al or P) over the other (P or Al) is between 5 and 75%, preferably between 5 and 50%.

Further, during the experiments of the present invention, it was found that Al was somewhat more effective at melting rare earth elements. Therefore, the molar amount of Al in the preferred composition is 7 to 75% more preferably 5% than the molar amount of P.
Those which are larger by ~ 50% are preferred. On the other hand, in applications with high levels of ionizing radiation and high levels of optical power, it is preferred to have a high P level and a P-rich composition is preferred.

The structure of the fiber laser according to the present invention is as follows.
It has a double clad layer structure to facilitate end pumping or clad layer pumping. A single cladding layer structure (also side-pumped) is also applicable to the present invention. In the present invention, the cladding layer
A layer that partially performs a light guiding function. In a double clad layer structure, the second clad layer also functions as a protective layer, and when performing this function is also referred to as fiber coating. However, a fiber laser having a double clad layer structure may have a coating layer other than the second clad layer.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a fiber laser device.

FIG. 2 is a sectional view of a fiber in the fiber laser device of FIG. 1;

[Explanation of symbols]

 Reference Signs List 11 optical fiber coil 12, 13 plug reflector 14 laser diode 15 output 21 core 22 first cladding layer 23 second cladding layer

Continuation of front page (71) Applicant 596077259 600 Mountain Avenue, Murray Hill, New Jersey 07974-0636 U.S.A. S. A.

Claims (8)

[Claims] 1. A length of glass having a core (21), a first cladding layer (22) surrounding the core, and a second cladding layer (23) surrounding the first cladding layer. (B) reflection means (12, 13) for forming a laser cavity along the glass fiber at both ends of the glass fiber (11); and (c) one end of the glass fiber. And a laser means (14) arranged so that its output matches the core of the glass fiber. The composition of the core of the glass fiber (11) is 0.1 to 2.0 mol%. Rare earth element, 0.2 to 3.0 mol% of Ge, 0.5 to 8.0 mol% of Al, 0.5 to
8.0 mol% of P and the remainder are SiO ₂ , and the difference between the refractive index of the core (21) and the refractive index of the first cladding layer (22) is 0.008 or less. Fiber laser. 2. The fiber laser according to claim 1, wherein said first cladding layer (22) is made of at least 85% or more of SiO ₂ . 3. The mole percent of Al is 5 to 5 mole percent greater than the mole percent of P.
2. The fiber laser of claim 1, wherein said fiber laser is 75% larger. 4. The mol% of P is 5 to 5 mol% of Al.
2. The fiber laser of claim 1, wherein said fiber laser is 75% larger. 5. The fiber laser according to claim 1, wherein said core has a diameter of 5.5 μm or more. 6. The rare earth element includes Tb, Nd, Ho,
2. The fiber laser according to claim 1, wherein the fiber laser is selected from the group consisting of Dy, Tm, and Er. 7. The fiber laser according to claim 5, wherein said rare earth element is Yb. 8. The fiber laser according to claim 1, wherein the difference between the refractive index of the core (21) and the refractive index of the first cladding layer (22) is 0.072 or less.